
\begin{tikzpicture}[scale=.8]
  \coordinate[label=left:$A$] (A) at (0,0);
  \coordinate[label=right:$B$] (B) at (7,0);
  \coordinate[label=above:$C$] (C) at (5,4);
  \pic[mark angle={blue}{3mm}{1.2}, "$\alpha$"] {angle=B--A--C};
  \pic[mark angle={blue}{3mm}{1.2}, "$\beta$"] {angle=C--B--A};
  \pic[mark angle={blue}{3mm}{1.2}, "$\gamma$"] {angle=A--C--B};
  \draw (A) -- node[below] {$c$} (B)
    -- node[right] {$a$} (C) -- node[above] {$b$} cycle;
\end{tikzpicture}
